1. Recombination in low-bandgap InGaAs   Tim Gfroerer Davidson College, Davidson, NC with Mark Wanlass National Renewable Energy Lab, CO ~ Supported by Bechtel Bettis, Inc. and the American Chemical Society – Petroleum Research Fund ~

2. Experiments by . . . Colleen Gillespie (Davidson ’06) Pete Campbell
Patten Priestley (Davidson ’03)
and Malu Fairley (Spelman ’03)

3. Motivation: TPV converters
0.0
0.2
0.4
0.6
0.8
1.0
T=1300 C
Normalized Intensity
0.0
0.5
1.0
1.5
2.0
2.5
Energy (eV)
Increasing the Indium concentration in the InGaAs lowers the bandgap and
increases the fraction of blackbody radiation that is absorbed in the cell.

4. Variable-Bandgap Lattice-Mismatched Stuctures
Undoped InAsyP1-y, 30 nm
Undoped InxGa1-xAs, 1.5 μm
Undoped InAsyP1-y buffer, 1 μm
Undoped InAsyP1-y step-grade region:
0.3 μm/step (~ -0.2% LMM/step), n steps
Undoped InP substrate
Nominal sample structure: not to scale.

5. Efficiency Measurements
CW YAG laser nm
ND Filters
Variable Temp Cryostat
Sample
Photodiode
Lowpass Filter
: Laser Light
: Luminescence
Experimental set-up and definition of radiative efficiency.
light in = heat + light out
radiative efficiency = light out / light in
heat
light in
light out

6. Defect-Related Density of States
ENERGY
Valence Band
Conduction Band
The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above.

7. Radiative Efficiency Measurements
light
heat
Radiative efficiency measurements at 77K. The theoretical fits correspond to
the defect-related DOS functions indicated in the inset graphs.

8. Defect-Related Transition Probabilities+ + +
The probability P of transitions involving phonon emission depends on the number of phonons required, which is determined by the position of the defect level in the gap.

9. Temperature Dependence
Temperature dependence of radiative efficiency vs. excitation, showing how
the SRH and Auger mechanisms depend on Indium concentration.

10. Auger Recombination
Temperature and bandgap dependence of the Auger coefficient C. The CHSH
(band-to-band) mechanism dominates Auger recombination in low-bandgap InGaAs.

11. Sub-Bandgap Photoluminescence
FTIR spectra showing a deep transition in the lattice-matched material that
abates and then disappears with increasing [In].

12. Four Conclusions Three References
Deep defect levels → shallow near-bandedge states with increasing [In].
The CHSH Auger mechanism is dominant in this alloy.
Sub-gap PL from deep (Ea > 0.2 eV) levels ↓ and then disappears with increasing [In].
Structure-less sub-gap cathodoluminescence supports assignment of this band to point defects.
Three References
T.H. Gfroerer, L.P. Priestley ('03), F.E. Weindruch ('01), and M.W. Wanlass, APL 80, 4570 (2002).
T.H. Gfroerer, L.P. Priestley ('03), Malu Fairley (‘03), and M.W. Wanlass, JAP 94, 1738 (2003).
T.H. Gfroerer, C.E. Gillespie (‘05), J.P. Campbell (‘03), and M.W. Wanlass, JAP 98, (2005).